observability-architect

Observability Architecture — Signal Production

What an application emits about itself: logs, metrics, traces. Pairs with [grafana-architect](../grafana-architect/SKILL.md) which owns the consumption side (dashboards, alerts). This skill enforces what gets emitted, how it's named, and how the three pillars correlate. Wiring code in [RECIPES.md](RECIPES.md); pinned libraries in [STACK.md](STACK.md).

1. Three pillars + correlation

Each pillar answers a different question:

| Pillar | Question | Cardinality | Cost | |---|---|---|---| | Logs | "What happened on this request?" | High (per event) | High storage | | Metrics | "What is the rate / aggregate?" | Low (aggregated) | Low storage | | Traces | "What was the path?" | One per request | Medium |

The design goal is correlation: from any signal you can pivot to the others. A trace shows a slow request → click → see the logs from that request → see the metric spike around the same time. This is what makes observability worth the cost.

Correlation mechanism: trace_id everywhere.

• Every log line carries the current trace_id and span_id.
• Every metric exposes the current trace_id as an exemplar (Prometheus exemplars).
• Every trace span carries the operation name, attributes, and status.

If you skip the correlation, you have three independent data lakes — useful in isolation, painful to cross-reference. Concrete wiring in [RECIPES §1](RECIPES.md#1-correlation-trace_id-in-log-lines).

2. Metrics: Prometheus

Why Prometheus over OTel metrics in 2026: Prom client libs are mature in every language, the exposition format is universal, ops engineers already know rate() / histogram_quantile(). OTel metrics are catching up but not yet at parity for ergonomics.

Naming

Follow the Prometheus / OpenMetrics convention:

• Pattern: <namespace>_<subsystem>_<name>_<unit>_<type>
• <namespace> = application name (orders, payments).
• <unit> is always present for sample values: _seconds, _bytes, _total, _ratio, _celsius. Never _ms, never bare units.
• <type> suffix for cumulative counters: _total. Gauges and histograms don't take a type suffix.

orders_http_requests_total{method="POST", route="/v1/orders", status="201"}
orders_http_request_duration_seconds_bucket{le="0.5", route="/v1/orders"}
orders_db_connections_active        # gauge, no suffix

Types

• Counter — monotonically increasing; _total suffix; reset on process restart. Query with rate().
• Gauge — point-in-time value, up or down. Active connections, queue depth, in-flight requests.
• Histogram — distribution of values; choose buckets explicitly. Default latency buckets in [RECIPES §4](RECIPES.md#4-default-histogram-buckets).
• Summary — pre-computed quantiles; avoid unless you can't aggregate across instances (you usually can — use histogram).

Cardinality

The single most expensive observability mistake: high-cardinality labels.

• Bad: user_id, email, request_id, correlation_id, anything unique per request. One time series per unique value — millions of series, ruined retention.
• Good: method, route (templatized, not the raw path), status_class (2xx, 4xx, 5xx), tenant, region. Bounded sets.
• Templatize routes to bounded patterns: /v1/users/{id} not /v1/users/01J9X....
• < 100 distinct values per label as a soft cap; review labels with more.

3. Logging: structured, leveled, correlated

• Structured JSON to stderr (per [cli-tool-architect §6](../../tooling/cli-tool-architect/SKILL.md#6-output-discipline--stdout-for-data-stderr-for-logs)). One log line = one JSON object.
• Stdlib first:
• Go: log/slog per [go-architect §5](../../languages/go-architect/SKILL.md#5-stdlib-defaults). slog.JSONHandler in production.
• Python: structlog configured to emit JSON.
• Mandatory fields: timestamp (RFC 3339 UTC), level, msg (fixed string), trace_id/span_id (when in a traced request), service.
• Don't log full payloads. A request body field can be PII; the bytes are dead weight even when it isn't. Log the shape: request_size_bytes, field_count, customer_id.
• msg is a fixed string for grep-ability: msg="user created" with user_id and email_domain as separate fields, not msg=f"created user {email}".
• One level per environment: prod info, staging info/debug, dev debug.
• Errors include the operation that failed + the inputs that mattered. No stack traces in msg; stack traces are a separate error.stack field.

4. Tracing: OpenTelemetry

OTel for traces is unambiguously the right choice — vendor-neutral, well-instrumented per language, supports all major backends (Jaeger, Tempo, Honeycomb, Datadog).

• Auto-instrumentation first. OTel libs for net/http, gin, FastAPI, requests, psycopg, grpc cover 80% of useful spans for free. Add manual spans only where business boundaries deserve them.
• Span naming: verb + resource. POST /v1/orders, db.query, kafka.publish. Templatized, low cardinality.
• Attribute keys follow OTel semantic conventions: http.method, http.status_code, db.system, messaging.system. Don't invent your own.
• Set span status on errors: span.SetStatus(codes.Error, msg) — backends colorize error spans.
• Don't trace everything. Per-call spans for hot loops kill performance. Trace the request boundary, major sub-operations (DB call, external API call, queue publish), and failure paths.

5. Correlation rules

Three rules to make signals jump between each other:

1. trace_id in every log line. Pull from the OTel SDK's current context — every modern logging integration supports this.
2. Prometheus exemplars on histograms (especially latency). When a slow bucket increments, the exemplar records the trace_id. Grafana lets you click from a histogram bucket directly to the trace.
3. service.name and service.version as resource attributes on traces + as labels on metrics + as fields on logs. Ties signals across deploys and versions.

Wiring snippets in [RECIPES §1](RECIPES.md#1-correlation-trace_id-in-log-lines).

6. Sampling

• Head sampling at 10% in production. Deterministic per trace_id so a request is either fully sampled or fully dropped — no half-traces.
• 100% sampling on errors — always keep the trace when span status is error. Cheap, decisive ROI.
• 100% sampling in non-prod (dev, staging) — volume is low; you want full visibility.
• Tail sampling via the OTel Collector is the upgrade when 10% head misses interesting low-volume endpoints. Adds Collector infrastructure. Defer until needed.
• Configure via env vars — see [RECIPES §5](RECIPES.md#5-sampling--env-var-configuration).

7. What NOT to emit

PII and secrets never appear in any signal:

• No email addresses, names, addresses, phone numbers in attributes or fields. Use email_domain (acme.com) to slice by tenant.
• No passwords, tokens, API keys, session cookies — not in headers, not in payloads. Use redaction filters at the SDK level — defense in depth, since one careless log.Info("creating user", user) can leak it all.
• No full payloads. Log size and shape, never contents.
• No internal IDs that could be re-identified. UUID v7 IDs are fine in metric labels (they don't leak meaning per [sql-architect §1](../../databases/sql-architect/SKILL.md#1-schema-design)); raw auth_token_id is not.

Configure your SDK redaction list at startup; review it on every deploy.

8. SLOs and golden signals

What to measure for every service.

RED — for request-driven services

| Signal | Question | Metric | |---|---|---| | Rate | How many requests/sec? | <svc>_http_requests_total over time | | Errors | How many fail? | `<svc>_http_reque